Photographic processing wastes arising from the processing of black-and-white and color photographic materials are usually divided into two classes-those containing silver ion eluted from the photographic materials in an appreciable concentration (silver-present class) and others (silver-absent class), for the purpose of recovery of silver as a valuable metal. Silver is recovered from the silver-containing class by the trade concerned in the disposal of waste liquid. In general the exhausted processing solutions issuing as wastes from the fixation step, and the bleaching and the combined bleaching and fixing steps for processing color photographic materials are grouped into the silver-present class, and the exhausted processing solutions issuing as wastes from the development step are grouped into the silver-absent class. On the other hand, the exhausted processing solutions issuing as wastes from the washing and the stabilization steps are grouped into either silver-present or silver-absent class depending upon their respective silver ion concentrations.
Since photographic processing wastes contain high concentrations of various kinds of inorganic and organic compounds, they cannot be discharged into a sewer or a river as they are. Accordingly, it is necessary for them to be treated to some extent.
Under these circumstances, a number of treatment methods have been studied to make feasible the discharge of such wastes into a sewer or a river. Specifically, known treatment methods for reducing the pollution loads of photographic processing wastes include activated sludge methods [as disclosed, e.g., in JP-B-55-49559 (The term "JP-B" as used herein means an "examined Japanese patent publication"), JP-B-51-12943, JP-A-48-13265 (The term "JP-A" as used herein means an "unexamined published Japanese patent application") and JP-A-50-2353], evaporation methods (as disclosed, e.g., in JP-A-49-89347 and JP-B-56-33996), electrolytic oxidation methods (as disclosed, e.g., in JP-A-48-84462, JP-A-49-119458 and JP-B-53-43478), ion exchange methods (as disclosed, e.g., in JP-B-51-37704, JP-B-53-383 and JP-B-53-43271), reverse osmosis methods (as disclosed, e.g., in JP-A-50-22463) and chemical treatment methods (as disclosed, e.g., in JP-A-53-12152, JP-B-57-37396 and JP-A-61-241746). However, each of these methods have their individual defects as described below.
As for the activated sludge methods, for example, there is difficulty in employing the method disclosed in JP-B-55-49559, wherein the treatment of wastes is carried out under aeration using concentrated oxygen gas (O.sub.2 &gt;20.9%), to the treatment of wastes with high concentrations corresponding to biological oxygen demand (abbreviated as "BOD" hereinafter) of 3,000 ppm or more, because it is impossible to finish efficiently the decomposition of BOD and the chemical oxygen demand ("COD") components within an appropriate time by subjecting the wastes to subsequent secondary treatment unless the initial BOD of the wastes is controlled to 3,000 ppm at the most. JP-A-50-2353, on the other hand, discloses an oxidation treatment for thiosulfate-containing wastes, wherein the aeration is carried out under the condition that the thiosulfate-containing wastes are mixed with a sulfur oxidizing bacteria containing solution and the pH adjusted to 4-9 and, what is more, the concentration of nitrogenous compounds is controlled to 0.1% or lower, as based on nitrous acid. This treatment, however, has a problem when it is applied to a photographic fixation waste, which is a type of photographic processing waste, because disorganization of the sludge occurs or the treatment efficiency is insufficient. Moreover, although activated sludge methods have low operation cost, they have little effect upon ingredients which are not biodegradable. In particular, activated sludge methods essentially fail in disposal of chelating agents, such as ethylenediamine-tetraacetic acid (EDTA) and the like, present in quantity in photographic processing wastes.
As for chemical treatment methods, the treatment of wastes is known to be carried out by the addition of hydrogen peroxide, persulfates, perhalogenates, halous acids or hypohalous acids. However, all of these methods have very low efficiency in treating photographic processing wastes with high COD values, so that these reagents are always used in large excess resulting in an increase in operational cost.
As for the ion exchange methods and the reverse osmosis methods wherein resins or membranes are used, frequent renewal of the resins or the membranes is required since they become adsorbed or contaminated with ingredients which tend to polymerize, such as developing agents. In these methods also, the operational cost tends to increase. Moreover, in treating concentrated wastes from photographic processing solutions, the resins and the membranes become greatly fatigued, so that it becomes impossible to use them in a short period of time.
As for the electrolytic oxidation methods, they have the following problems in spite of their strong oxidizing power: (1) in the oxidative decomposition of wastes with high COD, an increase in equipment cost and an increase in treatment time occur because a large quantity of electric current is required; (2) pollution of electrodes due to high molecular weight compounds occur because organic compounds present in the wastes, such as a developing agent, tend to polymerize; (3) evolution of poisonous gases such as hydrogen sulfide occurs upon decomposition of lower sulfur compounds such as thiosulfates; (4) insufficient reduction of BOD occurs because the decomposition of organic compounds stops at a level of lower fatty acids with a BOD load, such as acetic acid, propionic acid and the like; and so on.
In order to provide simple and technically easy treatment of photographic processing wastes, the number of steps must be reduced to a minimum and to the addition of special chemicals is to be avoided. In addition, desirably the wastes produced in separate steps should be treated collectively, because photographic processing consists of several steps and the separate treatments of the wastes produced in the respective steps creates difficulties. In particular, collective treatment is desirable for small-scale photofinishing laboratories.